1. Field of the Invention
The present invention relates to a semiconductor system, in particular, a semiconductor system that works as a thyristor in an on-state, and as a bipolar transistor in a transient state during an on/off transition or a state when an overcurrent flows.
2. Description of the Related Art
Recently, among power semiconductor devices, a voltage-drive bipolar device combines an advantage that it has a MOS gate drive structure made of a metal-oxide-semiconductor, which makes it convenient in the controllability to the drive thereof, and another advantage that a saturation voltage is low owing to bipolar operation. Accordingly, such a device has had an expanding range of applications. Furthermore, since a power semiconductor device is used as a noncontact switch, it is better to be low in the occurrence loss. The occurrence loss is in many cases expressed with an index of saturation voltage (on-state voltage drop) verses switching loss trade-off characteristics (hereinafter, simply referred to as trade-off characteristics).
As one of the most effective methods for improving the trade-off characteristics, there are a method in which a base region of a device (drift region) is made thinner and a method in which a concentration of carriers in the vicinity of a surface of the device is made higher. In the former method, the device is proposed to have a FS (field stop) structure. As the latter method, for instance, in the case of an IGBT (insulated gate bipolar transistor), which is a main stream among the voltage drive bipolar devices, a method in which a current path on a surface of a semiconductor is made narrower and thereby a hole concentration in a base region is raised is applied.
Furthermore, as still another method, a method of forming into not a voltage drive-type bipolar transistor such as an IGBT but a voltage drive-type thyristor such as an MCT (MOS controlled thyristor), DGMOS (dual gate MOS thyristor) or EST (emitter switched thyristor) was actively studied in the 1990s. Still furthermore, as a composite system, a composite system of a MOSFET (metal-oxide-semiconductor field effect transistor), a diode and an electrostatic induction thyristor and a composite system of a MOSFET, a diode and an EST are proposed.
For example, Japanese Utility Model Application Laid-Open No. 06-31230 discloses a composite semiconductor system in which a first switching element is cascode-connected to an electrostatic induction thyristor and a constant voltage element is connected between a gate electrode of the electrostatic induction thyristor and the first switching element. In the semiconductor system a second switching element is provided in parallel with the constant voltage element for bypassing a gate current of the electrostatic induction thyristor when the electrostatic induction thyristor is turned-off. According to the Japanese Utility Model Application Laid-Open No. 06-31230, the composite semiconductor system combines characteristics of low power loss with excellent high-frequency characteristics.
Japanese Patent Application Laid-Open No. 07-302897 discloses an insulating gate thyristor that includes a first conductivity-type base layer, a second conductivity-type base layer formed on a surface of the first conductivity-type base layer, a first conductivity-type emitter layer formed in the second conductivity-type base layer, a second conductivity-type emitter layer formed at a place different from the second conductivity-type base layer in the first conductivity-type base layer, a gate electrode disposed through an insulating film on a surface of the second conductivity-type base layer interposed between the first conductivity-type base layer and the first conductivity-type emitter layer, and a first electrode disposed on a surface of the second conductivity-type emitter layer. The insulating gate thyristor also includes a semiconductor switching element that is dielectrically isolated from the first conductivity-type base layer and connected to the first conductivity-type emitter layer, a second electrode connected to the semiconductor switching element, and a semiconductor rectifying device disposed between the second conductivity-type base layer dielectrically isolated from the first conductivity-type base layer and the second electrode. According to Japanese Patent Application Laid-Open No. 07-302897, the on-characteristics and the turn-off capability of the insulating gate thyristor can be improved to realize a larger current system.
However, although the abovementioned existing devices or systems all have trade-off characteristics superior to those of the IGBT, there are disadvantages in that these lack the current saturation characteristics and short circuit capacity that the IGBT has, and a reverse bias safe operating area (RBSOA) is narrow. Accordingly, it cannot be said that these devices or systems are popular products at present. Furthermore, in some voltage drive thyristors such as MCCTs (MOS Controlled Cascode Thyristor), the abovementioned disadvantages are overcome. However, since an IGBT structure is disposed in parallel with a thyristor structure, it is difficult sufficiently to extract an effect of raising a hole concentration in a base region, owing to the thyristor operation.
The invention, in order to overcome the abovementioned problems involving the existing technologies, provides a semiconductor system that includes a voltage drive-type thyristor or a semiconductor device excellent in the trade-off characteristics. Furthermore, the invention provides a semiconductor system that includes a voltage drive-type thyristor or a semiconductor device having a high short circuit capacity.